Ordinary digital-to-analog converters (DAC'S) provide a discrete output level for every value of a digital word that is applied to their input. There is difficulty in implementing these converters for long digital words because of the need to generate a large number of distinct output levels. A method for circumventing this difficulty calls for spanning the signal range with a few widely spaced levels and interpolating values between them. The interpolating mechanism causes the output to oscillate rapidly between the levels in such a manner that the average output represents the value of the input code. This technique provides a trade off between the complexity of the analog circuits and the speed at which they operate.
Essential to the technique is an interpolating circuit for truncating the input words shorter output words. These shorter words change their value at high speed in such manner that the truncation noise that lies in the bandwidth of the signal is satisfactorily small.
The above method is disclosed in the following references: (1) "Interpolative Digital-to-Analog Converters" a paper by Messrs. G. R. Ritchie, J. C. Candy, and W. H. Ninke, published in Vol. COM-22, No. 11, November 1974 of the IEEE Transactions on Communications; and, (2) U.S. Pat. No. 4,006,475 issued Feb. 1, 1977 to Mr. J. C. Candy et al.
The aforesaid method uses a simple accumulation of the truncation error to perform interpolation. A problem with this method, however, is that the truncation noise, sometimes referred to quantization noise in the prior art, needs to be reduced. It has been an objective, then, to skew the spectral noise curve in such a way that the noise is substantially moved to high frequency, out of the desired signal band. It has been suggested that the accumulator used for interpolation in the aforesaid Candy patent be replaced by a digital filter. Indeed, such a digital filter is disclosed in U.S. Pat. No. 4,467,316 issued Aug. 21, 1984 to Mr. H. Musmann et al. As shown by curve S.sub.q " in FIG. 4 therein, it is desirable to move the noise out of the signal band to the right.